Semiconductor devices such as logic and memory devices are typically fabricated by a sequence of processing steps applied to a specimen. The various features and multiple structural levels of the semiconductor devices are formed by these processing steps. For example, lithography among others is one semiconductor fabrication process that involves generating a pattern on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated on a single semiconductor wafer and then separated into individual semiconductor devices.
Optical inspection processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield. Optical inspection techniques offer the potential for high throughput without the risk of sample destruction.
Optical apertures with differing shapes and sizes are employed to manipulate illumination and imaging properties in optical systems. In particular, optical aperture shape and size may be tuned to increase the measurement signal to noise ratio for a specific wafer pattern, defect of interest, etc.
In some examples, an iris mechanism with adjustable blades, e.g., a traditional camera shutter mechanism, is employed as an adjustable aperture mechanism. The aperture size is varied by moving the blades between an open position and a closed position. Although the size of the aperture is fully adjustable, the shape of the aperture is determined by the size, and is limited to an approximately circular shape that scales in size as the blades move between the open and closed positions.
In some other examples, a rotating wheel including multiple apertures is selectively positioned such that a desired aperture shape is positioned in an optical beam path. However, the number of aperture shapes and sizes is limited by the number of predetermined apertures available on a particular wheel.
In some other examples a linear slider mechanism includes multiple apertures selectively positioned such that a desired aperture shape is positioned in an optical beam path. Again, the number of aperture shapes and sizes is limited by the number of predetermined apertures available on a particular linear slider. In some examples, a tape drive mechanism positions a desired aperture shape printed on a thin tape. The tape is positioned by rotating reels on both ends of the tape. Similarly, the number of shapes that can be presented by the tape drive mechanism is limited by the number of predetermined apertures available on the particular tape.
Liquid crystal arrays may be employed as programmable aperture mechanisms. However, liquid crystal arrays suffer from a number of practical limitations. Firstly, liquid crystal array pixels are not fully transmissive when programmed in a transmissive mode. This leads to excessive light loss. This is particularly costly in the context of a modern inspection system. Secondly, liquid crystal array pixels are not fully occluding in a blocking mode. This leads to excessive light leakage that is detrimental in a modern optical inspection system. Moreover, liquid crystal arrays may also induce stray light, scattered light, and optical aberrations that further degrade optical inspection performance. Furthermore, liquid crystal arrays may also suffer from limited lifetime when exposed to short wavelength light, particularly at high power densities.
Tilt mirror arrays may also be employed as programmable aperture mechanisms. However, tilt mirror arrays also suffer from light loss and optical artifacts such as stray light, scattered light, and optical aberrations that limit optical inspection system performance.
In some examples, aperture patterns have been exposed and developed on photofilm. However, photofilm systems, are not completely transmissive and completely occlusive, leading to undesirable light loss and light leakage. Moreover, exposure and development of photofilms is costly and time consuming, and the resulting films have a low damage threshold. Photofilm systems may also cause contamination of other elements of UV, DUV, and VUV light systems commonly employed in modern inspection systems.
In some other examples, aperture patterns have been applied onto transmissive substrates by inkjet printing. However, inkjet printing systems are not completely transmissive and completely occlusive, leading to undesirable light loss and light leakage. Moreover, inkjet systems require either a consumable substrate or a substrate that must be recleaned, leading to cost and throughput issues. Printed substrates may also cause contamination of other elements of UV, DUV, and VUV light systems commonly employed in modern inspection systems.
In some other examples, Fourier filters (e.g., a plurality of metal bars with adjustable spacing) have also been employed as flexible optical apertures. U.S. Pat. No. 5,970,168, which is incorporated herein by reference in its entirety, describes an example of a Fourier filter. However, Fourier filters allow for very few shapes and are typically employed for the limited purpose of blocking diffraction patterns.
In some other examples, a microshutter array, such as the array of microelectromechanical shutters employed as part of the James-Webb space telescope, may also be employed as a programmable aperture mechanism. However, microshutter arrays may suffer from excessive light loss in a transmissive mode due to the array structure. In addition, the arrays structure may induce stray light effects. Moreover, microshutter arrays are complex devices that give rise to cost and reliability issues.
As described hereinbefore, previous flexible optical aperture systems have several disadvantages such as optical transmission losses, incomplete optical blocking, stray light, optical aberrations, limited shape flexibility or spatial resolution, low damage threshold, and incompatibility with short wavelength light. Thus, methods and systems for improved programmable aperture mechanisms operable in modern optical inspection tools are desired.